Numerous vehicles have been used industrially which either have very high frames or which require high lift height capabilities when a lift axle suspension system is employed. Military vehicles, for example, can often have longitudinal frames which far exceed the ordinary height levels experienced on most commercial vehicles, and thus require special suspension systems extending between the frame member and the axle. In the private commercial field, an excellent example of a vehicle requiring high lift capabilities, particularly in the tag (behind the drive axle) position, is a cement mixer truck. It is known that such trucks may advantageously use a lift axle suspension system which extends beyond the rearward extremity of the longitudinal frames of the vehicle, thereby to provide a tag axle for lowering into engagement with the road surface when the cement mixer is loaded, but which may be raised beyond the level of the vehicle's frame.
Examples of cement mixers using tag axles in the aforementioned position include such patents as U.S. Pat. Nos.:
3,704,896 PA1 3,895,818 PA1 4,082,305 PA1 4,084,833
(See also the "Bridge Master" model by McNeilus Truck & Mfg. Inc. of Dodge Center, Mn.)
Tag axles of the type described are useful in that they allow additional weight carrying capacity in the vehicle, thus allowing maximized loads to be hauled. And, when such suspension systems are provided with lifting capabilities, the systems may be raised from the roadbed to provide maximized weight transfer to the drive axles for tractive effort.
Whether referring to specialized vehicles with high frames (the axle being used in any position, pusher, drive or tag) or whether referring to commercial vehicles wherein there is a need for an axle where high lifting capabilities are either desirable or necessary, there exists in the current technology a limitation on the travel that can be achieved. This is because the airbags (also referred to as air springs) are resilient, expandable members designed to carry a certain load in accordance with their cross-sectional area. If expanded too much, this decreases the safe load for which they were originally designed to carry because elongation decreases the effective cross-sectional area.
Given known commercial airbags of conventional design, the usual situation is that they can achieve somewhere between about 12-15 inches of travel when safely carrying a load of approximately 14,000 pounds. After this, the airbags narrow in diameter upon further expansion, to a point where their effective cross-sectional area becomes less than that desired for carrying the load. Where one desires more than 15 inches of travel, therefore, and particularly in those instances where upwards of approximately 30 inches of travel are desired for carrying loads of approximately 14,000 pounds, such axle suspension systems heretofore known, could not generally be used with maximized safety.
Concurrent with such problems in the industry, was a problem somewhat ubiquitous throughout the entire axle suspension field in commercial and other heavy duty trucks. This was the general inability to provide a convenient way in which to adjust caster on such vehicles. The problem was that such systems were usually of a trial and error type requiring either complex mechanisms on the one hand, or mechanical wedging devices on the other. An example of the latter can be found in the Lange High-Steer Model 6001 suspension system.
Further, and concurrent with the above, was the general need in the heavy duty trucking field for a beamtype suspension system capable of connecting an axle to the frame member of the vehicle, which provided both roll flexibility and lateral stability. Heretofore, it was felt that these two characteristics; namely, roll flexibility and lateral stability, conflicted in that it was previously believed that in order to design a suspension system properly, an engineering tradeoff had to be made between one characteristic and the other. Thus, suspension systems heretofore designed were often compromised below maximized roll flexibility in order to get appropriate lateral stability and vice versa.
The subject invention successfully solves the above-described problems in the art by providing a unique suspension system containing stacked airbags which are stabilized at their juncture to provide the desired safe ride characteristics, while achieving heights, including liftable heights, of up to about 30 inches. In addition, there is provided a unique and simple caster adjusting mechanism, as well as a unique beam-type suspension system capable of use in the overall suspension of this invention, but also useful in other suspensions, which achieves both roll flexibility and lateral stability without engaging in undue tradeoffs.